This invention relates generally to refrigerant heat pump systems and, more particularly, to parallel flow heat exchangers thereof.
A definition of a so-called parallel flow heat exchanger is widely used in the air conditioning and refrigeration industry and designates a heat exchanger with a plurality of parallel passages, among which refrigerant is distributed and flown in the orientation generally substantially perpendicular to the refrigerant flow direction in the inlet and outlet manifolds. This definition is well adapted within the technical community and will be used throughout the text. Parallel flow heat exchangers started to gain popularity in the air conditioning installations but their application in the heat pump field is extremely limited for the reasons outlined below.
Refrigerant heat pump systems typically operate in either cooling or heating mode, depending on thermal load demands and environmental conditions. A conventional heat pump system includes a compressor, a flow control device such as a four-way reversing valve, an outdoor heat exchanger, an expansion device, and an indoor heat exchanger. The four-way reversing valve directs refrigerant flown out of a compressor discharge port to either outdoor or indoor heat exchanger as well as routes it back to a compressor suction port from another of these heat exchangers, while the heat pump system is operating in the cooling or heating mode respectively. In the cooling mode of operation, the refrigerant is compressed in the compressor, delivered downstream to a four-way reversing valve and then routed to the outdoor heat exchanger (a condenser in this case). In the condenser, heat is removed from the refrigerant during heat transfer interaction with a secondary fluid such as air, blown over the condenser external surfaces by an air-moving device such as fan. As a result, the refrigerant is desuperheated, condensed and typically subcooled. From the outdoor heat exchanger, the refrigerant flows through the expansion device, where it is expanded to a lower pressure and temperature, and then to an indoor heat exchanger (an evaporator in this case). In the evaporator, refrigerant, during heat transfer interaction, cools air (or other secondary fluid) delivered to a conditioned space by an air-moving device such as fan. While the refrigerant, that is evaporated and superheated, cools the air flowing over the indoor heat exchanger, typically, moisture is also taken out of the air stream, thus the air is dehumidified as well. From the indoor heat exchanger, the refrigerant, once again, passes through the four-way reversing valve and is returned to the compressor.
In the heating mode of operation, the refrigerant flow through the heat pump system is essentially reversed. The refrigerant flows from the compressor to the four-way reversing valve and is routed to the indoor heat exchanger. In the indoor heat exchanger, which now serves as a condenser, the heat is released to the air to be delivered to the indoor environment by the fan to heat the indoor environment. The desuperheated, condensed and typically subcooled refrigerant then flows through the expansion device and to the downstream outdoor heat exchanger, where heat is transferred from a relatively cold ambient environment to the refrigerant, which is evaporated and generally superheated. The refrigerant is then directed to the four-way reversing valve and is returned to the compressor.
As known to a person skilled in the art, a simplified operation of the basic heat pump system has been described above, and many variations and optional features can be incorporated into the heat pump schematics. For instance, separate expansion devices can be employed for the heating and cooling modes of operation or an economizer or reheat cycle can be integrated into a heat pump design. Further, with the introduction of natural refrigerants such as R744, the high pressure side heat exchanger can potentially operate in the supercritical region (above the critical point), and a single-phase refrigerant will be flowing through its heat exchange tube instead of predominantly two-phase fluid such as at subcritical conditions. In this case, the condenser becomes a single-phase cooler type heat exchanger.
As can be seen from a simplified description of the heat pump operation, both heat exchangers typically serve a double duty as a condenser and as an evaporator, depending on the mode of operation. Further, a refrigerant flow through the heat pump heat exchangers is typically reversed (unless specific piping arrangements are made) during aforementioned modes of operation. Consequently, heat exchanger and heat pump system designers face a challenge to optimize the heat exchanger circuiting configuration for performance in both cooling and heating modes of operation. This becomes a particularly difficult task, since an adequate balance between refrigerant heat transfer and pressure drop characteristics is to be maintained throughout the heat exchanger. Therefore, many heat pump heat exchanges are designed with an equal, although not optimal, number of straight-through circuits for both cooling and heating modes of operation.
In general, the more vapor is contained in the two-phase refrigerant mixture flowing through the heat exchanger and the higher refrigerant flow rate the larger number of parallel circuits is required for efficient heat exchanger operation. Thus, the efficient condensers typically incorporate converging circuits and efficient evaporators employ either straight-through or diverging circuits. In other words, the heat exchanger circuits are either combined or split at some intermediate locations along the refrigerant paths to accommodate the changes in the refrigerant density and improve characteristics of condensing or evaporating refrigerant flows respectively. In conventional plate-and-fin heat exchangers, such circuit alterations, along with the refrigerant flow direction reversal, can be accomplished by utilizing the tripods and intermediate manifolds, as known in the industry. In the parallel flow heat exchangers, due to the design particulars as well as manifold design and refrigerant distribution specifics, the number of parallel circuits can be altered only at the manifold locations, restricting heat exchanger design flexibility, especially in the heat pump applications. Consequently, implementation of a variable number of parallel circuits along the heat exchanger length as well as variable length circuits for cooling and heating modes of operation represent a significant obstacle for heat exchanger and heat pump system designers and is not known in the art of parallel flow heat exchangers.
Another challenge a heat exchanger designer faces is refrigerant maldistribution, especially pronounced in the refrigerant system evaporators. It causes significant evaporator and overall system performance degradation over a wide range of operating conditions. Maldistribution of refrigerant may occur due to differences in flow impedances within evaporator channels, non-uniform airflow distribution over external heat transfer surfaces, improper heat exchanger orientation or poor manifold and distribution system design. Maldistribution is particularly pronounced in parallel flow evaporators due to their specific design with respect to refrigerant routing to each refrigerant circuit. Attempts to eliminate or reduce the effects of this phenomenon on the performance of parallel flow evaporators have been made with little or no success. The primary reasons for such failures have generally been related to complexity and inefficiency of the proposed technique or prohibitively high cost of the solution.
In recent years, parallel flow heat exchangers, and brazed aluminum heat exchangers in particular, have received much attention and interest, not just in the automotive field but also in the heating, ventilation, air conditioning and refrigeration (HVAC&R) industry. The primary reasons for the employment of the parallel flow technology are related to its superior performance, high degree of compactness and enhanced resistance to corrosion. As mentioned above, in the heat pump systems, each parallel flow heat exchanger is utilized as both a condensers and an evaporator, depending on the mode of operation, and refrigerant maldistribution is one of the primary concerns and obstacles for the implementation of this technology in the evaporators of the heat pump systems.
Refrigerant maldistribution in parallel flow heat exchangers occurs because of unequal pressure drop inside the channels and in the inlet and outlet manifolds, as well as poor manifold and distribution system design. In the manifolds, the difference in length of refrigerant paths, phase separation and gravity are the primary factors responsible for maldistribution. Inside the heat exchanger channels, variations in the heat transfer rate, airflow distribution, manufacturing tolerances, and gravity are the dominant factors. Furthermore, the recent trend of the heat exchanger performance enhancement promoted miniaturization of its channels (so-called minichannels and microchannels), which in turn negatively impacted refrigerant distribution. Since it is extremely difficult to control all these factors, many of the previous attempts to manage refrigerant distribution, especially in parallel flow evaporators, have failed.
In the refrigerant systems utilizing parallel flow heat exchangers, the inlet and outlet manifolds or headers (these terms will be used interchangeably throughout the text) usually have a conventional cylindrical shape. When the two-phase flow enters the header, the vapor phase is usually separated from the liquid phase. Since both phases flow independently, refrigerant maldistribution tends to occur, potentially causing the two-phase (zero superheat) conditions at the exit of some heat transfer tubes and promoting flooding at the compressor suction that may quickly translate into the compressor damage.
Thus, a designer of parallel flow heat exchangers for the heat pump applications faces the following challenges: implementation of the variable length diverging and conversing circuits for improving performance characteristics in the heating and cooling modes of operation, handling the reversed flow and avoiding maldistribution (as well as and other reliability issues such as oil holdup). Therefore, there is a need for improved parallel flow heat exchanger hardware and heat pump system designs which address and overcome the challenges described above.